HEATING-POWER. V^ 



is proportional to its specific gravity, so that heavy oakwood 

 produces more heat than lighter oakwood, and so on. Those 

 parts of a tree, therefore, which have a higher specific gravity 

 also possess greater heating-powers. Hence the heavy heartwood 

 of trees is more heating than the sap^vood, and rootwood is less 

 heating than wood from the stem, with the exception of the 

 highly resinous rootwood of some conifers. 



Since the heating-power of woods is greatly influenced by their 

 specific gravity, all local circumstances which increase the 

 specific gravity of wood will also increase its heating-power. 

 Thus, independently of the nature of the soil, the amount of 

 light which aff"ects the rate of growth and density of the wood of 

 a tree will increase its heating-power ; so that the broad-leaved 

 trees, which attain the best quality in this respect, are those 

 grown on southerly aspects and in fairly open woods, or with 

 perfectly isolated crowns, and not those growing on northerly 

 aspects and in dense woods. 



2, The Amount of Water hi the Wood. 



As long as the wood is not perfectly dry, it cannot, when 

 burning, produce its greatest possible heating effect, as much of 

 the heat is used for expelling the water in the form of vapour. 

 This is a matter of everyday occurrence, and it is clear that 

 splitting wood intended for fuel into small pieces and letting it 

 lie in the forest in airy places to dry, must increase the heating 

 power of any wood. 



Wood felled during spring or summer, and dried in the forest 

 at the warmest time of the year, is best in this respect. When- 

 ever it is intended to burn the wood as soon as possible after 

 felling, summer is the best season for the purpose ; but other- 

 wise the season of felling has no eft'ect on the heating-power of 

 wood. The anatomical structure of the wood also influences the 

 rate at which it dries, as soft and porous wood may be dried 

 more quickly and more thoroughly than denser wood. 



The influence the state of dryness of wood has on its heating 

 power is clearly shown in the case of barked oakwood, for 

 although oakwood, as a rule, burns slowly, barked oak coppice- 

 shoots, when thoroughly dry, burn as rapidly as the lightest 



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